The above research project is concerned with certain aspects of the enzymology of nitrate metabolism, namely, a continuing investigation at the molecular levels of (a) in vitro assembly of assimilatory nitrate reductase by incubation of a cell-free extract of the Neurospora nitrate reductase mutant (nit-1), induced by nitrate, and that of other mutants, uninduced wild type and other molybdenum-containing proteins. It includes the purification and characterization of both the nitrate-inducible protein of the nit-1 mutant and the presumed molybdenum-containing component or cofactor (including its ultimate identification), (b) Neurospora assimilatory NADPH-nitrate reductase, elucidating its subunit and cofactor interrelationships both structurally and functionally, (c) the purification and characterization of nitrate reductase-related proteins of several non-allelic nitrate reductase Neurospora mutants, and (d) the purification and characterization of the enzymes involved in the stepwise reduction of nitrate to ammonia (i.e., nitrate assimilation) and in nitrate respiration (where nitrate replaces molecular oxygen as the terminal electron acceptor) including denitrification. In addition to its academic value, the data could be helpful in evaluating the relationship between nutritive value and yield with respect to the use of different nitrogen compounds in agricultural practice. It may also prove to have a bearing on the normal and abnormal metabolic states of mammalian tissue in view of the known occurrence of the molybdenum cofactor in mammalian xanthine oxidase, liver aldehyde oxidase, and liver sulfite oxidase. BIBLIOGRAPHIC REFERENCE: Pan, S.-S. and Nason, A. Further Characterization of Purified Neurospora crassa Assimilatory NADPH-Nitrate Reductase. Fed. Proc. 35: In Press (1976).